The present invention relates generally to antennae loaded by a plurality meanderlines (also referred to as variable impedance transmission lines), and specifically to such an antenna providing multi-band operation.
It is generally known that antenna performance is dependent upon the antenna shape, the relationship between the antenna physical parameters (e.g., length for a linear antenna and diameter for a loop antenna) and the wavelength of the signal received or transmitted by the antenna. These relationships determine several antenna parameters, including input impedance, gain, and the radiation pattern shape. Generally, the minimum physical antenna dimension must be on the order of a quarter wavelength of the operating frequency, thereby allowing the antenna to be excited easily and to operate at or near its resonant frequency, which in turn limits the energy dissipated in resistive losses and maximizes the antenna gain.
The burgeoning growth of wireless communications devices and systems has created a significant need for physically smaller, less obtrusive, and more efficient antennae, that are capable of operation in multiple frequency bands. As is known to those skilled in the art, there is an inherent conflict between physical antenna size and antenna gain, at least with respect to single-element antennae. Increased gain requires a physically larger antenna, while users continue to demand physically smaller antennae. As a further constraint, to simplify the system design and strive for minimum cost, equipment designers and system operators prefer to utilize antennae capable of efficient multi-frequency and wide bandwidth operation. Finally, it is known that the relationship between the antenna frequency and the antenna length (in wavelengths) determines the antenna gain. That is, the antenna gain is constant for all quarter wavelength antennae (i.e., at that frequency where the antenna length is a quarter of a wavelength).
One prior art technique that addresses certain of these antenna requirements is the so-called xe2x80x9cYagi-Udaxe2x80x9d antenna, which has been successfully used for many years in applications such as the reception of television signals and point-to-point communications. The Yagi-Uda antenna can be designed with high gain (or directivity) and a low voltage-standing-wave ratio (i.e., low losses) throughout a narrow band of contiguous frequencies. It is also possible to operate the Yagi-Uda antenna in more than one frequency band, provided that each band is relatively narrow and that the mean frequency of any one band is not a multiple of the mean frequency of another band.
Specifically, in the Yagi-Uda antenna, there is a single element driven from a source of electromagnetic radio frequency (RF) radiation. That driven element is typically a half-wave dipole antenna. In addition to the half-wave dipole element, the antenna has certain parasitic elements, including a reflector element on one side of the dipole and a plurality of director elements on the other side of the dipole. The director elements are usually disposed in a spaced-apart relationship in the antenna portion pointing in the transmitting direction or, in accordance with the antenna reciprocity theorem, in the receiving direction. The reflector element is disposed on the side of the dipole opposite from the array of director elements. Certain improvements in the Yagi-Uda antenna are set forth in U.S. Pat. No. 2,688,083 (disclosing a Yagi-Uda antenna configuration to achieve coverage of two relatively narrow non-contiguous frequency bands), and U.S. Pat. No. 5,061,944 (disclosing the use of a full or partial cylinder partly enveloping the dipole element).
U.S. Pat. No. 6,025,811 discloses an invention directed to a dipole array antenna having two dipole radiating elements. The first element is a driven dipole of a predetermined length and the second element is an unfed dipole of a different length, but closely spaced from the driven dipole and excited by near-field coupling. This antenna provides improved performance characteristics at higher microwave frequencies.
The present invention discloses an antenna comprising one or more conductive elements, including a horizontal element and one or more vertical elements interconnected by meanderline couplers, and a ground plane. The meanderline coupler has an effective length that controls the electrical length and operating characteristics of the antenna. Further, the use of multiple vertical elements (each including one or more meanderline couplers) provides operation in multiple frequency bands.